Electrophoretic devices such as those illustrated in U.S. Pat. No. 6,392,785fall into the class of microencapsulated displacement types, and spinning or re-orientation types. In the former, charged submicroscopic particles contained within fluid filled transparent micro-shells, can be physically displaced under the influence of an electrical field. Depending on the color of the fluid and the color of the particles, a viewer looking at a sheet containing a dense array of such microshells will see a change in brightness from a dark state to a light state. In the latter, a permanent bipolar charge placed on microscopic sphere can be rotated under the influence of an applied field. If one side of the particle is black and the other white, the appearance of this device also changes with an applied voltage.
In both cases, the size of the particles, length of the required displacement, and viscosity of the supporting fluidic medium all contribute to relatively high voltages (approximately 30V-100V) required to drive the devices. Additionally, it is costly to incorporate color into the resulting media since this generally requires the addition of a costly color films. Speed is also an issue, for the aforementioned reasons.
Another similar approach, based on “liquid powder” offers a similar mode of operation. Analogous to the displacement version of the electrophoretic approach, this device relies on oppositely charged particles of opposite brightness that are physically displaced between two transparent electrodes. A change in brightness is the result. This change occurs at high speed because there is no liquid medium, the particles move through air, with response times of 100 microseconds achievable. High voltages of 80V-150V, due to the required large dimensions between electrodes, and costly color also constrain the capabilities of this device.
U.S. Pat. No. 6,215,920 describes an optical modulator whose primary optical principle is that of total internal reflection. A corner-cube reflector directs incident light back to the source by exploiting the total internal reflection (TIR) at the walls of the corner-cube. Particles which are brought into contact with the walls can spoil or degrade the TIR, thus reducing reflectivity significantly and the overall reflection of the structure. This approach, while offering the prospect of very high inherent reflectivities, does not incorporate a means for color selection. The design is further complicated by the tradeoff between positioning of the drive electrodes which could either degrade reflectivity (if located on the walls) or increase voltage (if located on the incident plane of the corner cube).